The ocean and atmosphere constantly exchange heat, momentum, and mass. This transfer, known as air-sea interaction, stems from a physically and chemically active air-sea interface that plays an integral role in the global climate system.
One of the processes driving the exchange between the atmosphere and water is the formation of sea spray. Breaking waves eject multitudinous water droplets, which enhance the interaction between the air and sea. Three physical processes are thought to create sea spray: wind shear, splashing, and popping of breaker-entrained air bubbles that reach the water surface. However, these processes have not been quantitatively linked to the creation of sea spray through experiments.
In a new study, Erinin et al. describe the formation of sea spray in experiments conducted in a freshwater wave tank using a programmable wave maker. They imaged the water droplets using a digital in-line holography system—a novel approach for studying sea spray generation—and used laser-induced fluorescence to produce surface profiles of the waves. The authors then synchronized the holographic movies and wave profiles to analyze the spray and how it formed.
The team’s findings offer new insights into both the size distribution of water droplets in sea spray and the space-time evolution of the droplet generation process. The imaging showed that each laboratory-scale breaking event produced hundreds of droplets ranging in radius from 50 to more than 1,400 micrometers (smaller droplets also formed but were below the size detection limit of the technique). The droplets formed during one of three distinct times: first, when the wave’s vertical jet impacted the free surface; next, when large air bubbles reached the surface and burst; and, finally, when smaller air bubbles reached the surface and popped toward the end of the wave-breaking process.
The research, which involved more than 140 experimental wave runs, offers a new, high-quality data set for the study of air-sea interactions—informing models of water droplet evaporation, for example—and provides profiles of sea spray, one droplet at a time. The authors plan to conduct additional experiments evaluating the effects of salinity, surfactants, and wind on sea spray. (Geophysical Research Letters, https://doi.org/10.1029/2019GL082831, 2019)
—Aaron Sidder, Freelance Writer